A belt retractor known from DE-U-296 05 803 comprises a belt reel rotatably mounted in a housing, a webbing reeled on said belt reel, a locking mechanism for blocking belt reel rotation, a planetary gearing including a sun gear connected non-rotatably to said belt reel, a ring gear secured to the housing, and at least one planet gear having an outer toothing, as well as a switch unit for determining the buckled-up condition on the basis of the length of the unwound webbing. The switch unit is electrically coupled to a control unit for activating a gas generator and includes a switch lever actuatable by the movement of the planet gear.
The known belt retractor has a planet gear which is coupled to a lever via a pinion rotatably mounted on the belt reel shaft so that the pinion rotates on orbiting of the planet gear about the sun gear. A toothing of a gear rack engages the toothing of the pinion. On unwinding of the webbing the pinion rotates with the planet gear, and the gear rack is moved in one direction. This shift in position of the gear rack is sensed for determining the buckled-up condition, i.e. whether the occupant has buckled up or not, whereby as of a predetermined length of the webbing unwound it is assumed that the occupant present in the vehicle seat assigned to the belt reel has in fact buckled up. It is only when the switch unit is actuated that the control unit activates in a restraint situation a gas generator which is part of a belt tensioner or a gas bag restraint system. Since the position of the gear rack is sensed via a spring-loaded switch lever in contact with the latter a relatively long path of force transmission materializes via numerous parts. These parts need to be in contact with each other at best with zero clearance so that reliable actuation of the switch unit is assured.